The mariners magazine, or, Sturmy's mathematical and practical arts containing the description and use of the scale of scales, it being a mathematical ruler, that resolves most mathematical conclusions, and likewise the making and use of the crostaff, quadrant, and the quadrat, nocturnals, and other most useful instruments for all artists and navigators : the art of navigation, resolved geometrically, instrumentally, and by calculation, and by that late excellent invention of logarithms, in the three principal kinds of sailing : with new tables of the longitude and latitude of the most eminent places ... : together with a discourse of the practick part of navigation ..., a new way of surveying land ..., the art of gauging all sorts of vessels ..., the art of dialling by a gnomical scale ... : whereunto is annexed, an abridgment of the penalties and forfeitures, by acts of parliaments appointed, relating to the customs and navigation : also a compendium of fortification, both geometrically and instrumentally / by Capt. Samuel Sturmy.

SECT. XL. How to give Level to a Piece of Ordnance, with the Gunner's Rule at any Degree of Random.

YOur Piece being Loaded in all points, as is before taught, and you have brought the Piece in a Right-line with the Mark, the Dispert being placed upon the Muzzle-Ring; in like manner place your Ruler upon the Base-ring, and let one standing by hold it, for the Foot of it fitted round to the Gun, you may be sure to put it right, and you may estimate on its perpendicular near enough; now having before the Distance to the Mark you intend to Shoot at; and admit you have found it to be 461 Paces, and the first Shot you made for Practice out of that Gun, conveyed her Shot at two degrees of Mounture 274 Paces, then according to the Rules in the 32 Section, and the Tables of Random, there I find 461 against 6 degr. which I must Mount the Gun to reach 461 Paces.

Then to find by this Table how many Inches, and hundred parts of one Inch 6 degr. will require; look in the Table above, and find on the left Hand in the first Column the length of the Piece 13 Foot just, under 6 degr. in the Common-Angle, you shall find 16 44/100 Inches, and to that height I set the Bead on the Lute-string, to 16 44/100 Inch, or 16 4/10; for every Inch is Divided into 10 Parts, and every Part is supposed to be Di∣vided into 10 more; then cause the Piece to be Mounted higher or lower, until you bring the Bead, the top of the Dispert, and the Mark all in one Line, stop the Piece in that position with a Coyn, Prime, and give Fire.

If you will Shoot by the Metal of the Piece, Substract the height of the Dispert out of the Inches found by the Table, and the Remainer, Mount your Piece unto; if the Dispert be 3 Inches ¼ long, Substracted from 16:44 found in the Table, leaves 12 19/100, or 122/50 of an Inch, you must set the height of the Bead to Shoot the same Di∣stance, by the Muzzle-Ring without the Dispert.